A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
Radiation used to irradiate the substrate in combination with contaminants—which may be introduced by, for example, outgassing of the photoresist layer on the substrate and also by ambient air entering the lithographic apparatus—may cause optical components in the projection system, for example, lenses, to degrade, which causes loss of overall transmission of the beam and loss of substrate illumination uniformity.
To address this problem, purging systems have been developed. The function of a purging system, for example, a purge hood, is to prevent or reduce chemical contamination of certain elements of the projection system and/or the illumination system. This may typically be done by blowing gas along an outer surface or surfaces of the elements in the projection system, so that the concentration of unwanted agents is diluted at the exposure slit by a factor of, for example, greater than typically 1000. However, dilution factors may vary between 100 to 10000 depending on, for example, the type of lithographic apparatus, the system specifications and the gas quality, etc.
In a purged system, while the internal elements in the projection system and the illumination system may be purged satisfactorily, since they are disposed in enclosed compartments, degradation continues to be a problem caused by the occurrence of contamination on, in particular, the surfaces of the first and last optical elements in the projection system, that is the first optical element encountered by the projection beam and the last optical element encountered by the projection beam as it passes through the projection system. Contamination of optical elements that may lead to the degradation of optical element performance includes, for example, dendritic salt structures which grow on the optical element surfaces. For example, optical elements subject to intense radiation over a period of time, typically a few years, become contaminated with salt structures. Thus, conventional purge hoods are typically positioned to provide cleaning gas along the surface of the outer bottom and top lens surfaces. Conventional purge hoods are mounted at a fixed position, for example, fixed to the projection system or a reference frame, such as the metrology frame. As mentioned, performance of the purge hood is typically expressed as a dilution factor, that is the ratio of the contaminants inside and outside the purged volume, and is conventionally of an order of 1000.
Conventional purge hoods, however, may suffer drawbacks. In particular, the performance of the purge hood may be negatively influenced by a gap between the purge hood and the substrate and/or between the purge hood and the substrate holder. As a result, the performance of the purge hood is dependent on the position of the substrate holder and is also influenced by gas showers provided to condition interferometric measurement components in the lithographic apparatus. For example, when no substrate holder is present, for example, when the substrate is being changed, the performance measured by the dilution may be poor. Further, dynamical disturbances, for example, flow induced vibrations, caused by the purge hood may affect the performance of the projection system. Conventional dilution factors may not be sufficient to reduce the moisture level at the optical element surface to well below 10 parts per million (ppm), which corresponds to less than one monolayer, which has been found to help prevent the formation of salt structures.
Further, a small gap between the substrate and the purge hood may not be satisfactorily achieved because of tolerances between the substrate table and the projection system. A similar situation may also be encountered in an immersion lithography apparatus, in which a small gap is to be maintained between a liquid supply system and the substrate, with at least part of the gap being filled with a liquid. In addition, the substrate table may move upward when servo control is lost, thus damaging the substrate when the gap between the substrate and the purge hood, or the liquid supply system is small.
Similar problems may arise between the illuminator and the support structure.